Towing a Subaru

I know you need a flat bed to transport a Subaru.

I park a lot in an area that has towing due to street cleaning and for other reasons. I usually manage to avoid getting caught, but every few years I get towed. No real problem with my last car, a Passat, aside from the fees. But what happens now that I drive a Subaru Forester?

Do all the tow truck drivers know about the Subaru requirement?
And if they do, do they care?
If the car was towed the normal way, would I easily be able to detect the damage, or would I just have a shorter lifetime on the differentials?

PS, no insult meant to tow truck drivers, I’m just a bit worried.

I’d love to have a magic device I can put on the car which would have a record of any towing… and record the differential slip (if that is the proper term).


Even if they do know they probably don’t care. If you complain that your car was damaged, the answer would likely be that you (the owner) are the cause of the damage by having your car illegaly parked.

^^ that ^^

In other words, I’m SOL…

Aside from the street cleaning towing, which is predictable, the other one I worry about is parking my car and the next day the city putting up a temporary no-parking sign and then towing the car.

With some of the stuff you have to worry about with Subarus, I don’t understand why people buy them. The rules about tires, and special fluids, and no towing… These are regular cars, but the rules are things you might expect from some specialized vehicle, not the family hauler Subaru appears to be.

“In other words, I’m SOL…”

In most cases…yes.

What should you do?
Don’t park when and/or where it’s banned.

With some of the stuff you have to worry about with Subarus, I don’t understand why people buy them. The rules about tires, and special fluids, and no towing…

I don’t see where any of these things are unique to Subaru. All AWD drive cars need similar tires. I’ve seen Chevy Blazers, Ford Explorers, BMWs with transfer case problems related to mismatched tires. Come to think of it, I think I’ve seen fewer problems with Subaru than other makes.

I can’t think of any specialized fluids that Subarus need either. At least not more so than any other car.

All AWD drive cars need similar tires. I’ve seen Chevy Blazers, Ford Explorers, BMWs with transfer case problems related to mismatched tires.

I’ve never understood why slight circumferential differences in tires should be a big deal. With the front/rear differentials, the planetaries have to do a little bit of extra turning. So what? They have to turn every time the car rounds a corner anyway.
With the center differential/viscous coupling, maybe a little bit of extra slippage added to all that from cornering. So what?
An unevenly loaded car, and even the visible difference in tire squishing front to rear from just engine weight ought to be just as significant I’d think.
What am I missing here?

I’ve asked this question before, trying to nail down why the coupling systems fail. Since a difference in speeds becomes dissipated by the viscous coupling system as heat energy, I wondered if the cooling of the coupling system is unable to dissipate heat efficiently enough to cope. I was hoping for a response from someone who had torn down a failed system, but nobody was able to offer insight.

I have the same question also.

Think of it this way: Different circumferential differences will mean that the wheels will be turning at different speeds as you go down the road. That’s not good for the AWD system as it causes differential and transfer case to wear internally. This damage does not happen overnight as it has a cumulative affect on the AWD. The traction systems on Subarus and Audis are very sensitive to mismatched tire sizes. It may not be as bad on some other models but I wouldn’t want to experiment to find out the answer.

So the same issue would be present between the two drive wheels in any FWD or RWD vehicle? But how often do you see a failed differential in a FWD or RWD vehicle?

Edit, although I had a 93 (?) Toyota which had two failed differentials.

I was hoping to hear from someone who’s torn some failed unit apart to find out whether the damage was heat damage to the fluid coupler or excess wear or heat damage to the differential gears. My gut tells me there’s something other than just excess gear wear involved.

One other possibility that occurred to me is that rear differentials use very high viscosity gear oils, which perform well under the high pressures that the teeth interfaces are subjected to. AWD systems don’t. That leaves me wondering if the actual root cause of the premature failures is that the lubricant used cannot tolerate the high pressures for extended periods.

I’d like to go beyond what parts wear out prematurely and find out WHY they wear out. I accept that such a failure analysis might never have been done (except, I’ll bet, by design teams at Subie), but I’m curious enough to ask. I would love to do a real root-cause failure analysis on a failed unit.

I do have a hint but no first hand knowledge of AWD failures. I hear that owners are reporting that their AWD systems “blew up” which may tell us that an extreme amount of heat is involved. I’d also like to know the exact reason that they fail but that information may be some time in coming.

This is an over-simplification of things, not all AWD systems operate the same, and I am by no means an AWD transfer case expert, but…

Different size tires on an axle will mean one side is rotating faster than the other. Given.

The gears in a standard open differential are designed to allow each axle to rotate at a different speed, because cars turn and the outer wheel will have to travel faster than the inner. Given. Put 2 different size tires on a standard axle and will you have problems? Yes. Maybe. 50,000 miles from now.

The gears in a posi-traction axle are designed to allow much less variation between the 2 axles. Enough so that if you don’t use the proper fluid even making a gradual turn will cause the gearset to chatter and bind. Put 2 different size tires on a posi axle and will you have problems? Yes. No one in their right mind would do that.

Now just turn that posi sideways, where the pumpkin is at the center of the car, one axle is now the front driveshaft and one axle is the rear driveshaft.

A good AWD system will not allow very much variation between the front and rear driveshafts, like a good posi diff will not allow much variation between the left and right wheels. That defeats the point of AWD. You want all 4 wheels turning at the same speed.

So different diameters front and rear will cause the AWD system to be in a constant state of slipping when driving on bare dry roads. The clutches will overheat, the fluid will burn, and that’s the beginning of the end.

OK @asemaster I understand your reasoning, and it appears to be logical. But why doesn’t driving on constantly curving roads do the same thing? There are very few roads in Vermont that are straight for very long, but Subarus are very common there, and there are lots of old ones still running. Every curve in the road creates the speed variance between the wheels that is supposed to cause excess wear. And isn’t that exactly the type of driving Subarus are made for?

Because driving on constantly curving roads the front wheels and the rear wheels are doing the same thing. There’s essentially no difference between the speeds of the front driveline and rear driveline.

Back in the old days when using 4WD meant getting out and locking the front hubs and shifting a big lever on the floor, you were always admonished to never use 4WD on bare dry pavement. That’s because the transfer case was run by a chain and gears that had no give. Today transfer cases have clutches or a viscous coupling. They’re a little more forgiving.

“With some of the stuff you have to worry about with Subarus, I don’t understand why people buy them…special fluids…”

I think that you have Subaru confused with Honda.
Hondas require a unique Honda-specific fluid for both the transmission and the differentials.
That is not the case with Subarus.